The invention relates to the field of fabricating injection molded parts. More precisely, the invention relates to an injection molding device and method for fabricating a part that make it possible to avoid potential deformation of the part relative to its expected nominal shape as can occur during solidification (e.g. polymerization) and as observed on extracting the part from the mold.
Existing injection molding methods include in particular the resin transfer molding (RTM) method in which a mold comprises two half-shells that, when placed one against the other, confine a mold cavity. A fiber preform is inserted into the cavity between the two half-molds and then resin is injected into the cavity. The resin is polymerized while keeping the two half-shells closed. Depending on the desired rate of production, polymerization may be performed at ambient temperature or by heating. Such a method can be used to make parts:                that are bodies of revolution, e.g. for fabricating gas turbine fan casings for aeroengines; and        parts that are flat, e.g. for fabricating rotor blades.        
The use of such a method is particularly advantageous since it makes it possible to obtain parts of overall weight that is less than the same parts would have if they were made of a metal material, while still presenting mechanical strength that is at least equivalent, if not better.
Nevertheless, such parts can also be made using other presently-known injection molding methods, such as plastic injection molding, or indeed molding metal in the context of foundry applications.
Whatever the molding method used, when the part is extracted from the mold, it can happen that it is observed to be deformed compared with its theoretical nominal shape. Thus, by way of example, the following might be observed:                for a body of revolution, such as a fan casing, there can be a defect relative to the desired circular shape, which is presented in the form of ovalization of the part on being extracted from the mold;        for a part that is flat, such as a rotor blade, on being extracted from the mold, there may be a departure from the expected angle of twist, e.g. a reduction in the value of the angle of twist, in other words the blade is warped.        
Such defects may be explained in particular by the fact that residual stresses apply to the part during its fabrication in the mold (e.g.: polymerization gradient, winding tension for a part made of composite material), which residual stresses are released when the part is extracted from the mold, thereby leading to deformation of the extracted part.
Generally, such defects are relatively constant and reproducible when using identical fabrication parameters.
Thus, in order to mitigate the problem of deformation in an injection molded part, one common solution consists in making a compensation mold having a mold cavity of a shape that does not correspond to the nominal shape of the part but rather a shape in which the deformation is taken into account so as to obtain, in the end, a part that has its nominal shape after the part has been extracted from the mold.
For example, in order to counter ovalization of a part that is expected to be circular when extracted, a compensation mold may be made in such a manner as to present a mold cavity that is ovalized in a direction perpendicular to a predetermined direction of ovalization that can be expected when the part is extracted from a mold having a cavity that is circular. While in the compensation mold the molded part thus presents a profile that is oval, prior to taking on a circular profile of the expected shape on being extracted.
Nevertheless, making a compensation mold of this type remains an operation that is complex and expensive. For example, the composite materials used for making mechanical parts present properties that depend on numerous factors (e.g.: three-dimensional weaving, winding tension, injection parameters, and polymerization parameters) that cannot always be completely determined from a theoretical point of view by the manufacturers of such parts.
It is thus often not possible to determine the shape of a compensation mold by calculation. Thus, in practice, making a compensation mold is the result of iterative testing, which leads to making a succession of several intermediate molds until the proper shape for compensation is determined empirically.
At present, using such a solution is found not only to be expensive, but also time consuming for producing molded parts.